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Honeycrisp, one of the most popular cultivars of apple Malus Ã- domestica Borkh., was developed in Minnesota and released in 1991 (Bedford, 2001). The fruit has become an important variety in the fresh fruit markets of the northern regions of the United States and Canada. Approximate 950,000 'Honeycrisp' trees had been planted in the U.S. with the largest number in Michigan and New York as of January 2001. The cultivar was also being grown in Europe, New Zealand, Australia and South Africa (Tong et al., 2003). Its unique texture, outstanding flavor, unusual crispness, and long storage-life (nine months) had commanded nice prices for growers and help revitalized the apple industry (DeEll, 2005; Delong et al., 2006).
The enthusiasm of industries for 'Honeycrisp', however, has been tempered by concern about the high incidences of quality-related storage disorders in fruit, like bitter pit, soft scald, soggy breakdown, water core, skin punctures, decay and internal browning, which vary in severity with the growing season and region (Evans, 2001; Greene and Weis, 2001; Schwallier 2001; Prange et. al., 2002). The first three disorders are the key factors to affect the fruit's market, thus the report will focus on the methods to reduce the incidence of bitter pit, soft scald and soggy breakdown. 'Honeycrisp' was extremely susceptible to bitter pit (Figure 1.) and usually can cause more than 50% of the fruit unmarketable (Dell, 2005). It could happen on the tree or develop during storage, especially with early harvest, delayed cooling, or warmer storage temperatures (Rosenberger et al., 2004). However, bitter pit could be managed by cultural practices such as calcium spray applications in the field because it's majorly caused by calcium-deficiency (Rosenberger et al., 2004). While the random incidence of soft scald and soggy breakdown in the fruit are the greatest concern for apple growers, which have resulted in great negative effects for the marketing of this cultivar.
Both soft scald and soggy breakdown were the low temperature injuries and should be worse at temperatures below 35 to 38 0F (Watkins et al., 2005). Soft scald (Figure 2.) was characterized by the development of sharply defined brown lesions on the apple peel, which could extend into the flesh, and the lesions were often invaded by the secondary infections (Watkins and Rosenberger, 2002; Moran et al., 2010). It was the most frequently observed storage disorder in Nova Scotia, sometimes reaching 100% incidence after harvest (Prange et al., 2002; Delong et al., 2006). Susceptible cultivars include 'Fuji', 'Jonathan', 'McIntosh', 'Delicious', 'Golden Delicious', and 'Honeycrisp'. Another common disorder is soggy breakdown (Figure 2.), which is an internal disorder. The apple flesh develops moist, brown, and spongy tissue (Watkins and Rosenberger, 2002; Watkins et al., 2004). Soggy breakdown developed majorly by inadequate ventilation of containers and storage rooms, a prolonged storage period, temperatures lower than 32 0F, variations in fertilizer treatments, and climatic conditions (Plagge and Maney, 1937). When some apples become immune to soggy breakdown, they should be more susceptible to soft scald. Sometimes both disorders could occur simultaneously on the same fruit (Plagge, 1929; Watkins et al., 2003).
Current Technologies to Control Soft Scald and Soggy Breakdown
In common storage (40-600F) or when fruit was stored at temperatures higher than 320F, soft scald and soggy breakdown did not develop. Since the industry had started to store apples at low temperatures of 30-320F, these two disorders began to be observed (Harley and Fisher, 1930). Up to now, they are the most common disorders in apple industries. Thus the low temperature plays a vital role in inducing soft scald and soggy breakdown. Considering the efficiency and economic issues for storing apples, low temperature storage will still be used commonly by producers. Under this condition, special postharvest treatments have been studied to prevent the storage disorders, which could reduce the losses for apple growers.
Storage Temperature Storage temperature is a critical factor in the development of storage disorders. Normally, apple was stored at temperatures above 32 0F. Fruit stored at 32 0F or lower develops more soft scald than fruit stored at 36 0F. The proper storage temperature for 'Honeycrisp' to control soggy breakdown is 38 to 40 0F rather than at lower temperatures (Schwallier, 2001; Nichols et al., 2004; Watkins et al., 2004). Soft scald and soggy breakdown development could be reduced when temperatures higher than 330F(Watkins et al., 2003).
Timing of Harvest The optimum harvest period for 'Honeycrisp' varied with the growing season and growing place (Wargo and Watkins, 2003). Fruits that harvested too early do not develop varietal flavor and are almost tasteless. If harvested too late, 'Honeycrisp' could develop fermentation products, such as ethanol and acetaldehyde, which caused undesirable flavors. Usually the precise harvest windows for each orchard will be identified using a combination of harvest index testing and visual observations. Prange et al. (2002) observed that soft scald occurs in 'Honeycrisp' fruit harvested early and late-harvested fruit having a 4-week storage period > 380F before CA storage at 380F developed very few disorders. Later harvest 'Honeycrisp' would be easier to develop soft scald and soggy breakdown in fruit (Watkins et al., 2003), while in Maine, harvested early could be prone to develop soft scald and soggy breakdown (Moran unpublished data). Ideal harvest time for 'Honeycrisp' would be about the second or third week of September in the Champlain region (Watkins et al., 2003).
Preconditioning Delaying the cooling of fruit prior to storage is a beneficial step in minimizing disorder incidence (Delong et al., 2004). Research at Cornell University and The University of Minnesota showed that a warm treatment (500F for 1 week) before cold storage resulted in the greatest reduction of soft scald (Bedford, 2001). Several recent reports indicated that a short, delayed cooling (or warming) period of 50 to 680F for 1 week prior to cold storage reduced the incidence of soft scald and soggy breakdown (Watkins and Nock, 2003; Delong et al., 2004; Watkins et al., 2004). 'Honeycrisp' apple treated with a delayed cooling regime consisting of 770F for 1-2d, or 860F for 1d, strongly suppresses or entirely eliminates the occurrence of both soft scald and soggy breakdown (Delong et al., 2009).
Controlled Atmosphere Storage (CA) Controlled atmosphere were usually defined as the content of 2.5kPa O2 and 1.0-1.5 kPa CO2. For many apple cultivars, an atmosphere of 2 to 3% CO2 with 2 to 3% O2 was thought to be suitable at cool storage temperatures. Controlled atmosphere storage could completely prevent Jonathan spot when only 2% CO2 was present and could reduce the incidence of core flush and various forms of flesh breakdown (Wills et al., 1998). CA storage was not recommended, because the great susceptibility of 'Honeycrisp' to CA injury had been detected by growers in different growing regions (Beaudry, 2009).
Diphenylamine (DPA) Watkins et al. (2004) have found that diphenylamine (DPA) could reduce soft scald development in some cases, but it was not effective enough to prevent the disorder in 'Honeycrisp' when stored at 33 0F. DPA did not have big effect on prevention of the soft scald and the impact on internal CO2 injury is in process (Beaudry, 2009). 'Honeycrisp' is not susceptible to superficial scald, so DPA has not been used by the industry to prevent soft scald (Moran, personal communication).
The above methods used to prevent or reduce soft scald and soggy breakdown were not always effective and could lead to other disorders and quality loss (Watkins et al., 2005), such as the effect of preconditioning on soft scald and soggy breakdown of 'Honeycrisp' was very inconsistent (Moran et al., 2010). Thus some new methods are in need to be investigated to better satisfy the customer demand.
Recently, several researchers had reported that 1-Methycyclopropene (Smartfresh, 1-MCP) could better prevent some storage disorders and maintain the fruit quality compared to previous methods. 'Red Delicious' treated with 1-MCP could get much higher levels of firmness and acidity when compared with control (Bates, 2001). 'Honeycrisp', 'Granny Smith', 'Golden Delicious', 'Fuji', and 'Braeburn' treated with 1-MCP had reduced ethylene production and respiration rate, slower loss of firmness and titratable acidity, slower degreening and yellowing, inhibition or reduced incidence of superficial and soft scald, core flush, and greasiness, as well as delayed production of ripe aroma (Argenta et al., 2001; Matthesis et al., 2002). Repeated weekly applications of 1-MCP (Mir et al., 2001; Mir and Beaudry 2001; Jayanty et al., 2004) helped prevent apple softening at 680F more than at 320F. 1-MCP not only had a dramatic effect on preventing or delaying the softening, also improved the fruit texture by increasing crispiness (Watkins et al., 2000; Mir et al., 2001; Mattheis et al., 2005; Moran and McManus 2005;). Apple storage at ambient temperature after treated with 1-MCP maintained better firmness than that cold storage, but the critical effect depends on the cultivars (Fan et al., 1999b; Mir et al., 2001). Green color loss in the ripening process could be prevented by 1-MCP treatment (Saftner et al., 2003; Zanella, 2003). 1-MCP could also reduce senescent breakdown (Watkins et al., 2000; Delong et al., 2004; Moran and McManus, 2005), core flush or brown core, and soft scald (Fan et al., 1999a; Zanella, 2003; Delong et al., 2004). The development of greasiness in some apple cultivars, such as "Granny Smith", was also a process that develops in cold storage, and this was inhibited by 1-MCP (Watkins and Nock, 2005). 1-MCP gas applied at harvest delays ripening, improved storage life and prevented superficial scald and other physiological disorders in many apple cultivars including 'Delicious', 'Granny Smith', 'Fuji' and 'Gala' (Fan et al., 1999a; Fan et al., 1999b).
1-MCP as a new technology, was reported to inhibit the ethylene action by preventing apple tissues from responding to ethylene that through the combination with ethylene receptor. This chemical compound had been thought as a promising breakthrough in apple storage technology that preserves the fresh-picked apple crunchiness, taste and juice content. Moreover, it could also help the apple industry meet retail and consumer demand for higher quality apples, as well as to deliver consistent quality to consumers year-round (Argenta eral., 2001; and Mattheisi et al., 2002).
'Honeycrip' treated with 1-MCP has great potential to improve fruit quality and reduce storage disorders during storage. This is because it has successfully improved other apple cultivars' quality, which as mentioned above. Another reason is that it has special characteristics which will beneficial fruit storage. Compared to other chemical methods, it is nontoxic to human and has activity at very low concentration (100ppb to 1ppm). When it is ready for a full commercial use, it will be packaged in a self-contained, one-step, single use delivery system. 1-MCP powder is very easy to be dissolved in water and release its active ingredients to storage room in a short time (Ian, 2003a; Bruce Bates, 2001). Ian (2003b) indicated that SmartFreshâ„¢ could easily complements the current systems of apple storage (CA and RA storage) and had benefited the South African apple industry. For most cultivars, treatment with 1-MCP was most effective when fruit were harvest at CA and 1-MCP was applied soon after harvest (Argenta et al., 2001; Matthesis et al., 2001). Using 1-MCP in storage 'Honeycrisp' apples could also reduce ethylene production, respiration, and greasiness (DeEll and Murr, unpublished data).
Apple treated with 1-MCP can improve many quality characteristics during storage, but few reports on the application of 1-MCP on 'Honeycrisp' have been published. Therefore the potential effect of 1-MCP on preventing storage disorders and improving fruit quality of 'Honeycrisp' is necessary to investigate.
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Figure 1: Bitter pit in 'Honeycrisp' (Photo by Dr. R.E. Moran)
Figure 2: Soft scald (left) and Soggy breakdown (right) in 'Honeycrisp' (Photo by Dr. R.E. Moran)